1. Field of the Invention
The present invention relates to a matrix type display unit, and more particularly to a matrix type display unit containing a drive circuit therein.
2. Description of the Related Art
The active matrix type display unit is a display unit in which a pixel is arranged at each intersection of a matrix which is made up of signal lines 1 and scanning lines 2, and a switching element is provided for each pixel in such a manner that pixel information is controlled by turning on/off the respective switching elements, as shown in FIG. 2. Liquid crystal 3 is used as a display medium of the display unit of this type. The switching element may be formed of, in particular, a three-terminal element, that is, a thin-film transistor 4 having a gate, a source and a drain.
Also, in the present specification, a xe2x80x9crowxe2x80x9d in the matrix is defined by the scanning line 2 (gate line), which is arranged in parallel to a subject row, being connected to a gate electrode of the thin-film transistor 4 of the subject row, and a xe2x80x9ccolumnxe2x80x9d in the matrix is defined by the signal line 1 (source line), which is arranged in parallel to a subject row, being connected to a source (or drain) electrode of the thin-film transistor 4 of the subject column. Furthermore, a circuit that drives the scanning line 2 is called a xe2x80x9cscanning line drive circuitxe2x80x9d, and a circuit that drives the signal line 1 is called a xe2x80x9csignal line drive circuitxe2x80x9d. Also, the thin-film transistor is called a xe2x80x9cTFTxe2x80x9d.
What is shown in FIG. 3 is a first conventional example of the active matrix type liquid-crystal display unit. The active matrix type liquid-crystal display unit of this example has the TFT using amorphous silicon, and the scanning line drive circuits and the signal line drive circuits which are made up of monocrystal integrated circuits (301, 303), and they are fitted onto the periphery of a glass substrate using tabs as shown in FIG. 3A, or the former are fitted onto the latter through the COG (chip on glass) technique as shown in FIG. 3B.
The liquid-crystal display unit of this type suffers from problems stated below. One problem may arise from the viewpoint of the reliability because the signal lines and the scanning lines of the active matrix are connected to each other through the tabs or bonding wire. For example, in the case where the display unit is of VGA (video graphic array), the number of signal lines is 1920, and the number of scanning lines is 480. The number of those lines shows a tendency to increase year by year as the resolution is improved.
In the case of producing a video camera view finder or a projector using liquid crystal, there is required that the display unit is compacted in a lump. The liquid-crystal display unit using the tabs as shown in FIG. 3A is disadvantageous from the viewpoint of a space.
There has been developed the active matrix type liquid-crystal display unit that solves those problems in which TFT is made of polysilicon. One example of this display unit is shown in FIGS. 4A and 4B. As shown in FIG. 4A, a signal line drive circuit 401 and a scanning line drive circuit 402 are formed on a glass substrate 400 together with pixel TFTs of an active matrix 403, using polysilicon TFTs. The formation of the polysilicon TFT is conducted by a high-temperature polysilicon process in which an element is formed on a quartz substrate through a process at 1000xc2x0 C. or higher, or a low-temperature polysilicon process in which an element is formed on a glass substrate through a process at 600xc2x0 C. or lower.
The polysilicon TFT can increase its mobility to 30 cm2/Vsec or more whereas the amorphous TFT is about 0.5 cm2/Vsec in mobility. Thus, polysilicon TFT can be operated by a signal of about severals MHz.
The drive circuit that drives the active matrix type liquid-crystal display unit is of the digital type and the analog type. The drive circuit using polysilicon is generally of the analog type. It should be noted that because the number of elements in the circuit of the digital type is remarkably more than that of the analog type, the drive circuit using polysilicon is generally of the analog type. Also, the circuit structure of the scanning line drive circuit and the signal line drive circuit generally uses a shift register 405 in which N-delay type flip flop circuits 404 are connected in series (refer to FIG. 4B).
The above-described conventional liquid-crystal display unit suffers from problems stated below. In the TFT using polysilicon, the control of a threshold value is generally difficult in comparison with a monocrystal transistor, and what is naturally to be of the enhancement type becomes of the depletion type so that a current may flow into a drain even though a voltage between a gate and a source is 0. This is because polysilicon is nonuniform in crystallinity more than monocrystal, a thermal oxide film cannot be used for a gate oxide film in the case of the low-temperature polysilicon, impurity contamination is caused, and so on.
For example, assuming that the TFT characteristic which is to be naturally exhibited by FIG. 5A becomes the characteristic shown in FIG. 5B with a shift of the threshold value, in an initial stage of an invertor circuit 600 shown in FIG. 6, no current flows when an input signal is in a high-state, but a current is caused to flow from a power supply to GND when the input signal is in a low-state. Further, current flows in the next stage in a high condition. Also, in the case where the drive circuit for the liquid-crystal display unit is installed in a substrate of a TFT, its stage number becomes 1120 in total at both of a signal side and a scanning side when the display unit is of the VGA type. As a result, even though a small current flows into each of the TFTs; the total value of the current becomes large. This causes a serious problem from the viewpoint of reducing a power consumption of the display unit.
On the other hand, if the threshold value becomes too large, an on-state current of the TFT is decreased, resulting in such a problem that the operating frequency of the drive circuit is lowered. The operating frequency of the drive circuit is determined by the magnitude of the on-state current when a load capacity and a supply voltage are kept constant because the load capacity is driven by the on-state current of the TFT. Hence, the too large threshold value leads to a lowered operating frequency.
The present invention has been made in view of the above problems with the conventional display unit, and therefore an object of the present invention is to provide a matrix type display unit that controls the threshold value of TFTs by the application of a voltage, thereby reducing a power consumption of a drive circuit or improving the operating frequency of the drive circuit.
In order to achieve the above object, according to a first aspect of the present invention, there is provided a matrix type liquid-crystal display unit, which comprises: a plurality of pixel portions which are arranged in the form of a matrix; a plurality of signal lines through which a display signal is supplied to said pixel portions; a plurality of scanning lines through which a scanning signal is supplied to said pixel portions; a drive circuit for driving at least one of said signal lines and said scanning lines; a plurality of thin-film transistors that form said drive circuit; and a threshold value control circuit being connected to said drive circuit for controlling a threshold value of said thin-film transistors.
According to a second aspect of the present invention, each of said thin-film transistors includes a control terminal through which the threshold value of said thin-film transistors is controlled, and said threshold value control circuit applies a desired voltage to said control terminal.
According to a third aspect of the present invention, said control terminal is formed in a channel contact region which is connected to a channel of said thin-film transistor, and said threshold value control circuit applies the desired voltage to said control terminal to change the channel, thus controlling the threshold value.
According to a fourth aspect of the present invention, the conductive type of said channel contact region is opposite to that of the channel of said thin-film transistors during operation thereof. Said channel contact region is p-type in case that the channel is n-type. Said channel contact region is n-type in case that the channel is p-type.
According to a fifth aspect of the present invention, said threshold value control circuit applies a voltage lower than a ground potential in order to reduce the power consumption of said drive circuit when said thin-film transistor is of the n-type.
According to a sixth aspect of the present invention, said threshold value control circuit applies a voltage higher than a supply potential in order to reduce the consumption power of said drive circuit when said thin-film transistor is of the p-type.
According to a seventh aspect of the present invention, said threshold value control circuit applies a voltage higher than a ground potential in order to improve the operating frequency of said drive circuit when said thin-film transistor is of the n-type.
According to an eighth aspect of the present invention, said threshold value control circuit applies a voltage lower than a supply potential in order to improve the operating frequency of said drive circuit when said thin-film transistor is of the p-type.
According to a ninth aspect of the present invention, said threshold value control circuit includes a variable resistor and adjusts the resistance of the variable resistor to apply the desired voltage to said control terminal.
According to a tenth aspect of the present invention, said threshold value control circuit includes a monitoring thin-film transistor that includes a threshold value control terminal for setting a reference value; a load for converting a current that flows in said monitoring thin-film transistor into a voltage; and an amplifier for amplifying a voltage developed across said load to apply an amplified voltage to said drive circuit, and to negatively feed back the amplified voltage to said threshold value control terminal of said monitoring thin-film transistor.
According to an eleventh aspect of the present invention, said threshold value control circuit is formed of a thin-film transistor on a substrate commonly used for that of said drive circuit.
According to a twelfth aspect of the present invention, said thin-film transistor is of a complementary transistor pair made up of an n-type transistor and a p-type transistor, the n-type transistor is provided with a first control terminal, the p-type transistor is provided with a second control terminal, and said threshold value control circuit applies desired voltages to the first and second control terminals, respectively.
According to a thirteenth aspect of the present invention, there is provided a liquid-crystal display unit, which comprises: a plurality of pixel portions which are arranged in the form of a matrix; a plurality of signal lines through which a display signal is supplied to said pixel portions; a plurality of scanning lines through which a scanning signal is supplied to said pixel portions; a signal-line drive circuit for driving said signal lines; a scanning-line drive circuit for driving said scanning-lines; a plurality of first thin-film transistors that form said signal-line drive circuit; a plurality of second thin-film transistors that form said scanning-line drive circuit; and a threshold value control circuit being connected to said signal-line drive circuit and said scanning-line drive circuit, for commonly controlling threshold values of said first and second thin-film transistors.
According to a fourteenth aspect of the present invention, there is provided a liquid-crystal display unit, which comprises: a plurality of pixel portions which are arranged in the form of a matrix; a plurality of signal lines through which a display signal is supplied to said pixel portions; a plurality of scanning lines through which a scanning signal is supplied to said pixel portions; a signal-line drive circuit for driving said signal lines; a scanning-line drive circuit for driving said scanning-lines; a plurality of first thin-film transistors that form said signal-line drive circuit; a plurality of second thin-film transistors that form said scanning-line drive circuit; a first threshold value control circuit being connected to said signal-line drive circuit, for controlling a threshold value of said first thin-film transistors; and a second threshold value control circuit being connected to said scanning-line drive circuit, for controlling a threshold value of said second thin-film transistors independently of said first threshold value control circuit.
According to a fifteenth aspect of the present invention, said first threshold value control circuit controls the threshold value so as to improve the operating frequency of said signal-line drive circuit, and said second threshold value control circuit controls the threshold value so as to reduce the power consumption of said scanning-line drive circuit.
In the liquid-crystal display unit of the present invention, the pixel portions are arranged in the form of a matrix, and there is provided the drive circuit for driving the signal lines through which the display signal is supplied to the pixel portions or the scanning lines through which the scanning signal is supplied to the pixel portions. The drive circuit is made up of a plurality of thin-film transistors. The drive circuit is connected with the threshold value control circuit for controlling the threshold value of the thin-film transistors. In the present invention, the threshold value control circuit is so designed as to control the threshold value of the thin-film transistors, thereby reducing the power consumption of the drive circuit or improving the operating frequency.
Each of the thin-film transistors is provided with the control terminal through which the threshold value is controlled. The threshold value control circuit applies to the desired voltage to the control terminal. Specifically, each of the control terminals is formed in the channel contact region which is connected to the channel of each thin-film transistor, and the threshold value control circuit applies the desired voltage to the control terminal to change the channel, thus controlling the threshold value.
The channel contact region is opposite in conductive type to the channel of said thin-film transistors. For example, when said thin-film transistors are of the n-type, the channel contact region is of the p-type. In this case, the channel contact region is formed by doping the region with p-type impurities. In this manner, the thin-film transistors each having the control terminal are formed. With such a structure, upon applying a voltage to the control terminal by the threshold value control circuit, the channel contact region functions as a so-called back gate, thereby influencing the channel of the thin-film transistor. As a result, the threshold value of the thin-film transistor can be controlled.
In this situation, the applied voltage is different between a case in which the power consumption of the drive circuit is to be reduced and a case in which the operating frequency is to be improved. Furthermore, the applied voltage depends on the polarity of the thin-film transistors. Specifically, when the thin-film transistors are of the n-type, a voltage lower than a ground potential is applied to the control terminal in order to reduce the consumption power of said drive circuit, or a voltage higher than the ground potential is applied to the control terminal in order to improve the operating frequency. On the other hand, when the thin-film transistors are of the p-type, a voltage higher than a supply voltage is applied to the control terminal in order to reduce the consumption power of said drive circuit, or a voltage lower than the supply voltage is applied to the control terminal in order to improve the operating frequency.
It should be noted that the control of the threshold value may be conducted by monitoring a current value of the drive circuit or a current value of the individual thin-film transistors, or automatically conducted by conducting the negative feedback. In the former case, the variable resistor is disposed in the threshold value control circuit so that the resistance of the variable resistor is adjusted, thus applying the desired voltage to the control terminal.
In the latter case, the threshold value control circuit may include the monitoring thin-film transistor for setting a reference value, the load for converting a current that flows in the monitoring thin-film transistor into a voltage, and the amplifier for amplifying a voltage developed across the load to apply an amplified voltage to the drive circuit and to negatively feed back the amplified voltage to the threshold value control terminals of the monitoring thin-film transistors. In the latter case, it is preferable that the threshold value control circuit is formed of a thin-film transistor on a substrate commonly used for that of the drive circuit.
Also, in the case where the thin-film transistors are of a complementary transistor pair (CMOS), the n-type transistor is provided with the first control terminal, the p-type transistor is provided with the second control terminal, so that the threshold value control circuit applies desired voltages to the first and second control terminals, respectively.
Also, the drive circuit includes the signal-line drive circuit for driving the signal lines, and the scanning-line drive circuit for driving the scanning lines. In this case, those drive circuits may be so designed as to be connected with one threshold value control circuit, to thereby commonly control the threshold values of the respective thin-film transistors, or the respective drive circuits may be so designed as to be connected with individual threshold value control circuits, to thereby control the threshold values of the respective thin-film transistors, independently. In particular, in the latter case, the threshold values of the respective thin-film transistors can be controlled by the first threshold value control circuit so as to improve the operating frequency of the signal-line drive circuit, and also they can be controlled by the second threshold value control circuit so as to reduce the power consumption of the scanning-line drive circuit. The reason why the threshold values are controlled independently is that the signal-line drive circuit and the scanning-line drive circuit are different in operating frequency. In other words, the operating frequency is more important to the signal-line drive circuit, whereas the power consumption is more important to the scanning-line drive circuit.